【作者】 陈少平；

【导师】 孟庆森；

【作者基本信息】 太原理工大学 ， 材料学， 2010， 博士

【摘要】 电场激活和压力辅助燃烧合成技术(Field Activated and Pressure Assisted Synthesis, FAPAS)是在多物理场(电场、机械场、温度场、化学场)耦合条件下的新材料合成新技术,具有温度高、升温速度快、高效和节能等优点,已经成为耐高温纳米块体材料合成的关键技术之一。将机械合金化(Mechanical alloying, MA)与FAPAS工艺用于一步法完成陶瓷的原位合成及其与金属的扩散连接,为异种材料的固相连接开辟了新途径。本研究采用MA-FAPAS工艺,借助中间层Ni3Al和TiAl的燃烧反应放热,一步完成(TiB2)pNi/Ni3Al/Ni、(TiC)pNi/TiAl/Ti和(TiB2-TiC)pNi/TiAl/Ti连接结构的梯度金属陶瓷层的合成以及各层之间的扩散连接。研究了不同放热体系、温度和电场强度等物理参数对材料燃烧合成过程和扩散连接界面形成机制的影响。利用最小Gibbs自由能原理对各反应体系的热力学特征进行计算;利用SEM、FE-SEM、TEM、EDS和XRD等手段对各层及连接界面的微观结构和相组成以及电场作用下各连接界面元素扩散特征进行分析;利用Thomas-Fermi方程对外加电场作用下异种材料连接界面扩散原子物态的变化进行计算分析;采用显微硬度压痕法和磨料磨损试验对合成梯度金属陶瓷层的断裂韧性和表明耐磨性进行分析;采用三点弯曲法、剪切法和冷淬法表征各连接界面结构特征对连接强度和抗热震性能的影响;利用ANSYS有限元数值模拟法分析各界面的应力应变和等效残余应力分布特征。研究认为,Ni-Al、Ti-Al、Ni-Ti-C和Ni-Ti-B4C不同反应体系的Tad和ΔG均随T0的增加而上升,在相应T0时具有获得目标产物的可行性。燃烧合成Ni3Al、TiAl和50wt%TiC-Ni的T0分别不低于600K、450K和600K,其余反应体系均可不进行预热;在对反应体系施加电场的条件下,对应的T0可以进一步降低。梯度金属陶瓷微观组织分析表明,合成的(TiB2)PNi、(TiC)pNi和(TiC-TiB2)pNi金属陶瓷均反应充分、组织致密且层间没有明显分界。以化学计量比混合的中间层Ni-Al粉体和Ti-Al粉体在燃烧合成中发生充分反应,分别形成细小均匀的单相Ni3Al等轴晶和TiAl等轴晶,并分别与梯度金属陶瓷层和金属层之间结合良好,连接界面处存在强烈的元素交互扩散,成分浓度呈梯度分布。连接界面扩散动力学分析认为,温度、电流和通电时间是影响各层间扩散连接界面形貌和结构的主要因素。随通电电流和时间的增大,Ni3Al/Ni和TiAl/Ti的扩散连接界面均变宽,且扩散层宽度的平方根与电流平方和时间的乘积呈线性关系。在外加电场和温度场耦合作用下,界面原子扩散由以晶格扩散机制为主转变为界面反应机制,最后为晶界扩散机制。增大电流和延长通电时间均会提高连接界面元素扩散系数并降低扩散激活能。外加电场对TiAl/Ti连接界面处Ti-Al交互扩散的影响大于对Ni3Al/Ni连接界面处Ni-Al交互扩散的影响,电场方向对Ti/TiAl/Ti连接结构的界面扩散动力学的影响存在差异性。外加电场作用下原子能量变化的计算分析认为,原子动能随着电流的升高而增大,外加电场通过增加扩散原子的动能促进原子扩散。力学性能分析结果表明,合成的三种金属陶瓷具有较高的硬度、表面耐磨性能和断裂韧性;三种结构的连接界面均具有较强的抗剥离性能和抗剪切强度,(TiC)PNi/TiAl/Ti连接结构的最大剪切强度为70.98MPa,(TiC)PNi/TiAl界面为接头的薄弱环节。ANSYS有限元数值分析结果表明,金属化合物中间层的存在有效减小了陶瓷和金属在冷却过程中产生的变形差异,三种连接结构中的热膨胀系数失配率分别为132.1%、10.8%和47.2%,相应产生的最大等效残余应力分别为38.74MPa、10.51MPa和17.93MPa。更多还原

【Abstract】 Field Activated and Pressure Assisted Synthesis, FAPAS, as a new process that is carried out in the condition of multi-fields including electric field, mechanical field, temperature field and chemical field, has become one of key technologies to prepare heat-resisting bulk materials with nano-structure for its characterization of high temperature, fast heating-up, efficiency and energy saving. A new approach is brought forward by the mechanical alloying assisted FAPAS process (MA-FAPAS) to have cermets in-situ synthesized and diffusion bonded with metals simultaneously.MA-FAPAS was employed to prepare (TiB2)pNi/Ni3Al/Ni ,(TiC)pNi/TiAl/Ti and (TiB2-TiC)pNi/TiAl/Ti joining structures by diffusion bonding and in-situ synthesis with Ni3Al and TiAl as exothermal media layer, respectively. The effect of different reaction system, temperature and current on the combustion synthesis and formation mechanism of the diffusion interfaces was analyzed. Thermodynamic properties of the different reaction systems were calculated based on the theory of the minimum Gibbs free energy. The microstructure and phases of each layer and the distribution of elements across the interfaces were determined by SEM, TEM, XRD, and EDS. The change of state of the atoms in interfaces was calculated by Thomas-Fermi equations under the action of the electric field. The hardness, tenacity, fracture toughness, and wear resistance of the surface of the synthesized gradient cermets were tested by means of micro-indent testing and friction testing. The bonding strength and thermal shock resistance were evaluated by the three-point bend testing, shearing testing and quenching testing, respectively. The residual stress in the boding interfaces was simulated by ANSYS finite element method.It is concluded that the adiabatic temperature Tad and Gibbs free energyΔG increase with T0 for Ni-Al, Ti-Al, Ni-Ti-C and Ni-Ti-B4C reaction systems, and it is feasible to obtain the target products by combustion synthesis. It is necessary to preheat not less than 600K, 450K, and 600K for Ni3Al, TiAl, and 50wt%TiC-Ni reaction systems, respectively, and that would be less when the electric field was applied.During the combustion synthesis, the powder elements for synthesis of the gradient cermets of (TiB2)PNi, (TiC)pNi and (TiC-TiB2)pNi get reacted completely to form compact products and no interface is observed between the gradient layers. The element powder mixtures of Ni-Al and Ti-Al with stoichiometric ratio got reacted completely to form Ni3Al and TiAl intermetallic with equiaxial polygonal grains and got bonded with the metal plates and gradient cermets well, respectively. The EDS analysis results showed that the elements across the interfaces distribute in grade by intense diffusion to each other.It is concluded by the diffusion kinetics that the temperature, current and time are the most important factors that determine the microstructure of the interfaces during the diffusion bonding. The thickness of the diffusion interfaces between Ni/Ni3Al and between TiAl/Ti increases with the current and time and its square root is in exact ratio to I2·t. Under the action of the electric field and temperature field, the movement of atoms in the interface is controlled by the lattice diffusion mechanism mainly at first and then shift to interface reaction mechanism and then to grain boundary diffusion mechanism. The coefficient of diffusion of the atoms increases and the activation energy of diffusion in the interfaces decreases with the increase of current and time during the diffusion bonding process.The effect of the electric field on the diffusion of TiAl/Ti bonding interface is stronger than that of Ni3Al/Ni bonding interface. The current promotes the atoms to migrate across interface by increase their kinetic energy. The direction of the current had different effect on the migrating of Al atom in the interface between Ti/TiAl/Ti.The results of the mechanical properties tests indicate that the in-situ synthesized cermets own excellent surface hardness, wear-resistance, and fracture toughness. The interfaces of the bonded structures had higher shearing strength and that of the (TiC)PNi/TiAl/Ti reaches to 70.98MPa.The analysis results of the residual stress of the samples simulated by ANASYS shows that the existence of the media layer of the intermetallic compound lowers the differences of the deformations between the layers. The mismatch rate of the coefficients of thermal expansion are 132.1%, 10.8% and 47.2%, respectively, and corresponding equivalent stress are 38.74MPa, 10.51MPa and17.93MPa。更多还原